Prepare the apples by peeling the skin, then slicing to remove the core. Slice the apples thin (approx 1/4 inch thick), then chop to 3/4 inch wide chunks (this makes it easier to eat than leaving apples in slices).
Place the diced apples in a medium size mixing bowl.
Add the brown sugar, flour, vanilla, cinnamon, nutmeg, salt, lemon juice and melted butter to the apples. Stir to combine.
Pour the apple mixture into a 9x13 or 3 quart baking dish. Spread to create an even layer.
In another mixing bowl combine the rolled oats, flour, light brown sugar, baking powder, cinnamon and salt. Stir this with a fork to combine.
Dice the cold butter into small squares, then add this to the topping mixture.
Using a pastry cutter, butter knives, or forks, cut the butter into the topping mixture until it has a coarse texture. Don’t leave chunks larger than 1/2 inch.
Sprinkle the topping mixture over the top of the apple mixture in an even layer.
Bake for 45 minutes or until topping is lightly golden brown and apple mixture is bubbling along edges.
Remove from oven and allow to cool for 5-10 minutes before serving.
Serve warm with vanilla ice cream.